In order to make the size of electronic instruments small, the heat generation density of power semiconductors, high-functional semiconductors, light emitting elements, and others has been increasing. For the heat dissipation of such electronic components, thermally conductive compositions are used. However, conventional thermally conductive compositions each produced by adding an inorganic filler to a resin have the following problems: when the content by percentage of the inorganic filler in the thermally conductive composition is high, the thermal conductivity is increased; however, the surface roughness of the thermally conductive composition becomes large and the formability thereof becomes low; and furthermore, the gloss of any surface of the thermally conductive composition lowers, and voids or the like increase in the surface or inside the composition.
The problems are more easily caused as the content by percentage of the inorganic filler is made larger. This is because the content by percentage of the resin component in the thermally conductive composition falls, and the inorganic filler is not easily wetted with the resin so as to be fixed.
As described above, as the content by percentage of the inorganic filler is made higher, flaws, cracks, voids and others are more easily generated in the surface as well as the formability deteriorates. Moreover, the inorganic filler itself gets low in cohesive force, or the inorganic filler exposed to the surface drop out easily. For this reason, even the fixation of electronic components and others onto the surface of the thermally conductive composition becomes difficult. If the volume fraction of the inorganic filler is, for example, over 66% by volume, it is very difficult to subject the thermally conductive composition to shaping or some other treatment. If the volume fraction of the inorganic filler is 66% or more by volume or further exceeds 70% by volume, the surface roughness Rmax is over 7500 Å. As the surface roughness Rmax is increased by an increase in the volume fraction of the inorganic filler, the glossiness abruptly drops down.
As described above, as the content by percentage of the inorganic filler becomes larger, the thermal conductivity itself gets larger. However, the surface roughness of the surfaces of the grains, which are actually shaped grains, abruptly becomes large so that the glossiness decreases abruptly. The matter that the surface roughness increases and the glossiness decreases means that the formability of the thermally conductive composition lowers, or innumerable voids are generated inside the thermally conductive composition or in the surfaces thereof.
Next, with reference to FIG. 34, a detailed description is further made. FIG. 34 is a schematic sectional view of a conventional thermally conductive composition when the content by percentage of an inorganic filler therein is a high value (of 66% or more by volume, or is a higher value of 70% or more by volume).
When the content by percentage of the inorganic filler is 66% or more by volume, innumerable voids 9 are generated in the front of free surface 8 or at the inside of thermally conductive composition 7, as illustrated in FIG. 34. Voids 9 cause an abrupt increase in the surface roughness of free surface 8 so that the glossiness declines. Free surface 8 means a natural surface which does not contact any other solid and further does not undergo polishing, cutting, or any other operation.
When the surface roughness of free surface 8 increases in this way and further voids 9 are generated at the inside or in the front thereof, the thermal conductivity of thermally conductive composition 7 is not increased very much even when the content by percentage of the inorganic filler is high. Moreover, the adhesiveness thereof onto a heat generator declines so that the thermal conductivity may lower. Furthermore, the formability of thermally conductive composition 7 deteriorates. Alternatively, thermally conductive composition 7 itself gets brittle to be easily cracked.
Against such problems, it is suggested to use, for example, a resin excellent in surface property and high in glossiness, such as a polyimide film, for thermally conductive composition 7, thereby improving the adhesiveness thereof to heighten a heat dissipating effect. As an application example thereof, a metallic core substrate is suggested. Next, with reference to FIG. 35, a conventional metallic core substrate is described. FIG. 35 is a sectional view of the conventional metallic core substrate.
Electrically insulating layer 11 is formed on metallic plate 10. Copper foil pieces 12 are laminated on electrically insulating layer 11. Solder pieces 13 are used to mount electronic component 14, semiconductor 15, terminal 16, and others thereon. As the thermal conductivity of electrically insulating layer 11 is higher, heat from electronic component 14 and semiconductor 15 can be conducted to metallic plate 10 so that a rise in the temperature thereof can be restrained. Electrically insulating layer 11 may be a matter in which an inorganic filler is added to a resin sheet in a film form (for example, Patent document 1).
As a method for increasing the thermal conductivity, a manner of using a material with high thermal conductivity as a filler in electrically insulating layer 11, or increasing the filling amount of the filler is often used. It is also effective to increase the thermal conductivity of a resin. With reference to FIGS. 36A to 36C, such a structure is described. As a means for increasing the thermal conductivity of the resin, for example, the use of a crystalline resin is suggested (for example, Patent document 2).
FIGS. 36A to 36C are explanatory views of a conventional crystalline resin. The use of the crystalline resin aims to polymerize molecules of monomer 18 having mesogen group 17 with each other to give electric non-conductance and excellent thermal conductivity. In order to use a crystalline resin to obtain a high heat dissipating performance (or a high thermal conductivity), it is necessary to increase the crystallization rate of the crystalline resin. As the crystallization rate of the crystalline resin is made higher, a substrate obtained therefrom is harder and more brittle. In other words, the substrate is not bent but broken, or is easily chipped or cracked. Thus, even when a crystalline resin is used to produce a heat dissipating substrate on which electronic components are to be mounted, the usage thereof is largely limited.
Even when an epoxy resin having mesogen groups is used, it is difficult to solve the problem described with reference to FIG. 34. As the epoxy resin is further crystallized, the problem described with reference to FIG. 34 may be more easily caused. Furthermore, such problems are easily caused from regions in which the content by percentage of an inorganic filler is lower than 66% by volume.
As a method for improving the strength, it is suggested that a method of blending a curing agent which can easily have a network structure, or some other method. However, in the case of a crystalline epoxy resin, the crystallinity thereof is inhibited when three-dimensional bonds are formed. As a result, a high thermal conductivity is not obtained in many cases.
Furthermore, a substrate in which a crystalline resin is used has a tendency that the substrate is hard and brittle, is not bent but broken, and is easily chipped or cracked. Thus, when a heat dissipating substrate is produced by use of a crystalline resin, there may remain a problem for impact resistance in a drop test or other tests.